Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. Still, with many hot gas path components constructed from super alloys, thermal barrier coatings (TBCs) can be utilized to insulate the components and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface, thus limiting the thermal exposure of the structural component.
While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight substrate materials have been proposed, such as ceramic matrix composite (CMC) materials. CMC and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) to protect them from the harsh environment of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment.
Silicon carbide and silicon nitride ceramics undergo oxidation in dry, high temperature environments. This oxidation produces a passive, silicon oxide scale on the surface of the material. In moist, high temperature environments containing water vapor, such as a turbine engine, both oxidation and recession occurs due to the formation of a passive silicon oxide scale and subsequent conversion of the silicon oxide to gaseous silicon hydroxide. To prevent recession in moist, high temperature environments, environmental barrier coatings (EBC's) are deposited onto silicon carbide and silicon nitride materials.
Currently, EBC materials are made out of rare earth silicate compounds. These materials seal out water vapor, preventing it from reaching the silicon oxide scale on the silicon carbide or silicon nitride surface, thereby preventing recession. Such materials cannot prevent oxygen penetration, however, which results in oxidation of the underlying substrate. Oxidation of the substrate yields a passive silicon oxide scale, along with the release of carbonaceous or nitrous oxide gas. The carbonaceous (i.e., CO, CO2) or nitrous (i.e., NO, NO2, etc.) oxide gases cannot escape out through the dense EBC and thus, blisters form. The use of a silicon bond coat has been the solution to this blistering problem to date. The silicon bond coat provides a layer that oxidizes (forming a passive silicon oxide layer beneath the EBC) without liberating a gaseous by-product.
However, the presence of a silicon bond coat limits the upper temperature of operation for the EBC because the melting point of silicon metal is relatively low. In use, a thermally grown oxide (TGO) layer of silicon oxide forms on the top surface of the silicon metal bond coat of a multilayer EBC system. This silicon oxide scale remains amorphous at temperatures of 1200° C. or lower, sometimes even at temperatures of 1315° C. or lower, although this property is also dependent on the time the bond coat is exposed to this temperature. At higher temperatures, or when minor amounts of steam penetrate through the EBC to the bond coat, the silicon oxide scale crystallizes (e.g., into cristoblate), which undergoes phase transition accompanied by large volume change on cooling. The volume change leads to EBC coating spall.
As such, it is desirable to improve the properties of a silicon bond coat in the EBC to achieve a higher operational temperature limit for the EBC.